Simple piezoelectric ceramic generator-based electrospinning apparatus

Abstract

The conventional high-voltage power supply (HVPS) used for electrospinning (e-spinning) is heavy and needs an extra electricity supply, which hinders the portability of the e-spinning apparatus. In this communication, we report on a new simple e-spinning setup using a piezoelectric ceramic (PZT) generator to replace HVPS, which makes the setup very simple, and may be the simplest e-spinning apparatus (small volume 5 × 1 × 1 cm³ and light weight < 10 g). The pulsed voltage generated by a PZT element is about 56 kV, which can meet the requirement for e-spinning. Three kinds of polymer solutions such as polyvinylpyrrolidone (PVP), poly(vinylidene fluoride) (PVDF) and polystyrene (PS) have been electrospun into ultrathin fibers successfully, which confirms the feasibility of this apparatus. The influences of different parameters such as e-spinning distance (1–9 cm) and solution concentration (11–14 wt%) were investigated. This work demonstrates a new approach to design a portable and simple e-spinning apparatus.

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1 Introduction

Electrospinning (e-spinning) has attracted much attention as an efficient and controllable method for nanofiber fabrication. Usually, an e-spinning setup consists of a high-voltage power supply (HVPS), a grounded metal collector and a solution container with a spinneret. The strong electrostatic forces between the spinneret and collector are used to produce ultrafine fibers from a polymer solution or melt through a vigorous whipping motion of charged jets due to fluid and electrically driven bending instabilities. The electrospun fibers have many potential applications in air filtration and liquid filters,¹ nano-electronic devices,^2,3 protective clothing⁴ and bio-medical materials (e.g., wound dressing,^5–7 tissue engineering,^8–11 and drug delivery^12,13) for their same remarkable advantages like large surface to volume ratio, controllable porosity, broad selection of polymer materials.^14–16 In recent years, electrospun fibers with different scales and constructions can be fabricated by modifying collector or auxiliary system. For example, the near-field e-spinning process provides a direct, continuous, and controllable manner to deposit solid nanofibers combined with an x–y movement stage.^17–21 The airflow-assisted e-spinning apparatus could produce a better outcome of in situ precise deposition, which had the potential to be used in wound dressing application.^22,23

However, these modifications made the e-spinning apparatus more and more complicated, which limits the applications of e-spinning especially in the case of outdoors or without power supply. To solve this problem, researchers have paid more attention to the replacement of HVPS and obtained some achievements. For example, a portable handheld e-spinning apparatus with volume of 10.5 × 5 × 3 cm³ and weight of about 120 g was reported by our group.²⁴ The hand-operated Wimshurst generator could also replace HVPS and could be used in solution e-spinning²⁵ and melt e-spinning.²⁶ In addition, Yan et al. reported a portable small solar cell-based e-spinning apparatus.²⁷

In this paper, we propose a much simpler e-spinning setup based on a piezoelectric ceramic generator as HVPS. The volume of this device can be reduced to 5 × 1 × 1 cm³, and the weight is less than 10 g. The voltage and the e-spinning performance of this device are investigated.

2 Materials and methods

2.1 Materials

Table 1 shows the details of different polymer solutions, which include the polymers and the corresponding solvents used for electrospinning in this work. Firstly, the polymers and solutions were weighted by an electronic balance and then stirred using a magnetic stirring apparatus. After stirring for three hours, the homogeneous polymer solution was achieved and then electrospun into fibers under room temperature.

Table 1 Details of different polymer solutions for e-spinning

Polymer	Molecular weight (g mol^−1)	Solvent	Concentration (wt%)	Viscosity (mPa s)
Polyvinyl pyrrolidone (PVP)	1 300 000	Anhydrous ethanol	11	650
			12	790
			13	980
			14	1070
Poly(vinylidene fluoride) (PVDF)	275 000	Dimethylformamide (DMF) : acetone (1 : 1)	14	220
Polystyrene (PS)	250 000	Tetrahydrofuran (THF)	18	110

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2.2 E-Spinning apparatus

Fig. 1a is a schematic diagram of the piezoelectric ceramic (PZT). The high voltage for e-spinning is supplied by one PZT, which can generate a very high pulsed voltage. Fig. 1b is the structural drawing of the new e-spinning apparatus based on a PZT generator. When the force is applied on the button, PZT element will be reshaped under the influence of stress. According to the piezoelectric effect,²⁸ the different charges are accumulated on two ends separately. The positive charges can be conducted to the spinneret by a metal lead and the negative charges can be connected to hand. Then a high voltage is generated between two electrodes. And this may meet the requirement for e-spinning. Fig. 1c shows a digital picture of this home-made portable e-spinning apparatus. The collector is an aluminum foil.

Fig. 1 (a) Structural drawing of a PZT, (b) schematic diagram of the new e-spinning apparatus based on a PZT generator as high-voltage power supply, and (c) digital picture of the home-made portable e-spinning apparatus.

2.3 Characterization

The morphologies of the electrospun fibers were obtained by a scanning electron microscope (S4700, Hitachi) and an optical microscope (BX-51, Olympus). The value of voltage generated by the PZT was measured by a digital oscilloscope (DSO-X 3024A, Agilent). And the viscosities of polymer solutions with different concentrations were measured by a viscometer (VT-04F, RION).

3 Results and discussion

3.1 High voltage measurement

The value of pulsed voltage generated by PZT decides the feasibility of e-spinning. Because of the discharging time is too short, a common voltmeter cannot measure the value of pulsed voltage. So we designed a bleeder circuit which included two resistors (1 kΩ and 1 MΩ) and a PZT generator just as Fig. 2a showed. In addition, the attenuation ratio of high voltage probe was set to 10× in experiment. And the wave shape of the resistor with a resistance of 1 kΩ can be shown in the digital oscilloscope when we operated piezoelectric ceramic generator.

Fig. 2 (a) Closed circuit designed for voltage measurement and (b) wave shape recorded by the digital oscilloscope.

As shown in Fig. 2b, twenty wave shapes were recorded by the digital oscilloscope. And the average pulsed voltage value is 5.61 V and the standard deviation is 0.31. We can know that the stability of PZT is good according to the standard deviation. By calculating, the pulsed voltage of one PZT generator is about 56 kV (56 kV = 5.61 V × 10 × 1001, namely, multiply voltage value (5.61 V) by attenuation ratio (10×) and magnification (1001)), which can meet the requirement for e-spinning.

3.2 E-Spinning distance

The SEM images of PVP fibers obtained under different e-spinning distances (1–9 cm) are shown in Fig. 3a–e. The average diameter of PVP fibers was measured by the SEM analysis and the statistical histogram is shown in Fig. 3f. According to Fig. 3f, we can see the average fiber diameter reduced from 0.97 μm to 0.705 μm when spinning distance increased from 1 to 5 cm. The reason can be ascribed to a longer (lengthened) fiber tensile curing time for a longer spinning distance in the electric field. And after that, the average diameter of PVP fibers displayed an increasing trend from 0.705 μm to 1.104 μm when the distance increased to 9 cm. This change can be due to the weakening of electric field intensity and the electrostatic drawing forces of the charged jets become weaker. This change is similar to that of the conventional e-spinning setup. And the optimized spinning distance of the new setup is about 5 cm.

Fig. 3 SEM images of the PVP fibers (12 wt%) produced under different e-spinning distances: (a) 1 cm, (b) 3 cm, (c) 5 cm, (d) 7 cm, and (e) 9 cm. (f) Statistical histogram of the average fiber diameter.

3.3 Different concentrations of PVP spinning solutions

Fig. 4 displays the SEM images of the PVP fibers prepared under different concentration of spinning solution in the range from 11 wt% to 14 wt%. As shown in Fig. 4a–d, we can see that the surface of fibers is smooth, which demonstrates the success of each e-spinning process.

Fig. 4 SEM images of the electrospun PVP fibers prepared under a spinning distance of 5.0 cm, different concentration of PVP spinning solution: (a) 11 wt%, (b) 12 wt%, (c) 13 wt%, and (d) 14 wt%.

The linear statistical graph of average diameter of PVP fibers is shown in Fig. 5. We can see clearly an increasing trend of average fiber diameter from 0.655 μm to 1.068 μm when the PVP concentration of spinning solution increased from 11 wt% to 14 wt%. This is because that the concentration of spinning solution decides the viscosity. And the viscosity increases from 650 to 1070 mPa s with the increasing of PVP concentration from 11 to 14 wt%, as shown in Table 1. The higher concentration leads to higher viscosity. So the viscosity resistance becomes higher while the electric filed force is constant. As a result, the resultant force on charged polymer jet becomes weaker. And the charged fibers are stretched deficiently and the average fibers diameter increases with the increasing of concentration. If the concentration (or viscosity) is too high, e-spinning will be prohibited by an inability to control and maintain the flow of polymer solution to the spinneret.

Fig. 5 Statistical histogram of the average fiber diameter.

3.4 E-Spinning of other materials

We also tried to prepare fibers of other polymer materials such as PVDF and PS by using this simple e-spinning apparatus. As shown in Fig. 6a–b, ultrathin PVDF (14 wt% concentration of PVDF) and PS (18 wt% concentration of PS) fibers can be produced successfully by this new setup (only one PZT generator used) under a spinning distance of 5.0 cm. Fig. 6c–d show their corresponding fiber diameter distribution maps. The average diameter is about 0.973 μm for the PVDF fibers and 0.866 μm for the PS fibers. The results demonstrate the feasibility and universality of the new portable apparatus.

Fig. 6 SEM images of the electrospun (a) PVDF and (b) PS fibers produced under a spinning distance of 5.0 cm; corresponding fiber diameter distributions for the electrospun (c) PVDF and (d) PS fibers.

4 Conclusion

In conclusion, we have proposed a new portable e-spinning setup based on a piezoelectric ceramic generator (which can generate a pulsed voltage of 56 kV) as high-voltage power supply. This portable device is very simple and may be the simplest e-spinning apparatus. By this apparatus, ultrafine fibers of PVP (average fiber diameter of 0.655–1.104 μm), PVDF (0.973 μm) and PS (0.866 μm) were produced, which could confirm the feasibility of this apparatus. Experimental parameters such as spinning distance (1–9 cm) and PVP concentration (11–14 wt%) were also investigated. It was found that the optimizing spinning distance was about 5 cm, the increase of PVP concentration could result in a larger fiber diameter. This new e-spinning apparatus provides an interesting idea to replace traditional high-voltage power supply and a new design to enhance portability and simplification of e-spinning setup.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51373082 and 31400851), the Taishan Scholars Program of Shandong Province, China (ts20120528), and the Postdoctoral Scientific Research Foundation of Qingdao.

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Footnote

† These two authors contributed to this work equally.

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